In this invention, the term “kinetics parameters” includes the reaction rate constants, active site concentration, and gas diffusivity. The terms “reaction rate constant” and “gas diffusivity” are used conventionally as defined and discussed in chemical kinetics texts. Here, the term “active site” is used in a narrower sense than is usual in describing powder catalysts as follows. “Active site” refers to an atom or a group of atoms on a solid catalyst where a reactant gas adsorbs fast enough to maintain a catalytic reaction cycle, an active site is defined experimentally by the reactant gas, and the active sites for different reactants may be different. “Active” means catalytic. The active site concentration is the amount of active sites per weight of catalyst powder.
The term “active site concentration” used here is different from the term “specific surface area” often used to describe catalyst powders. The latter means the entire surface. It is now understood that when a solid surface is used as a catalyst, for many reactions it is only a fraction of the surface that is active, not its entire exposed surface. However, there has been no means that can selectively measure just the active fraction. The methods used to normalize for the amount of catalyst or catalytic sites have to make do with the use of the entire surface. But, because the active fraction need not be the same for all catalyst surfaces, normalization by the entire surface can be misleading. The present invention meets the need for a means to selectively measure just the active fraction of the surface.
Powders in common use as catalysts in heterogeneously catalyzed processes are made by methods that give highly variable active site concentrations. The measurement of the active site concentration is useful for evaluating the efficiency of a preparation method. It is also useful for comparing the catalytic activity of different catalysts because the comparison is made on a more fundamental basis when the reaction rate is normalized by dividing by the active site concentration to give a comparison that uses the rate per active site.
As mentioned, the presently used method to normalize a reaction rate with respect to the used amount of catalyst actually uses the entire surface area of a powder. In the prior art methods, this entire surface area is determined by measuring the amount of gas adsorbed when the surface had been adsorptively saturated. The gas amount adsorbed, known as the monolayer amount, is converted into a surface area using the cross-sectional area of the adsorbed molecule. A commonly used method uses the pulsing of a series of similarly sized gas pulses, also known to those skilled in the art as the pulse or flow chemisorption method. This uses the decrease in the peak area, due to adsorption, of the chromatogram of the series of pulsed gas to determine the amount of adsorbed gas. This method has the defect that the measurement is carried out until repeated pulsing of the same sized pulse shows that consecutive pulses have the same peak area. This is a fundamental defect because the repeated pulsing provides for adsorption to saturation of the entire surface or multiple adsorption on the same site.
The present invention uses a different principle to determine the active site concentration when gases are pulsed that avoids these defects.
Other specific methods to determine the specific surface area include the method known as the BET measurement, which includes the static volumetric method, flow method, and gravimetric method, and the static volumetric method of chemisorption measurement, which are described in standard texts on the characterization of catalysts. It is a defect in all these methods that they perform their measurements until there is no further change in the gas phase concentration or pressure, which is their criterion that determines when adsorption saturation is reached. All these prior art methods may be inaccurate because this means to verify saturation requires the catalyst surface to be equilibrated with the gas phase for at least a few minutes and in many cases, some tens of minutes. The equilibration contact with a gas phase for a period of time has the defect that it causes the loss of an important distinction between sites, namely, the ability to distinguish sites of different adsorption energies or rates. The long equilibration times leads to the occupation of all the adsorption sites, including sites where adsorption is too weak or slow for a reasonable reaction rate; thus, it causes the loss of the distinction of sites with different adsorption energies or rates.
Another shortcoming of the prior art methods that measure the surface area is that they must use non-reactive conditions. This is because these methods use long equilibration times or a long series of pulses. This precludes the use of reactive conditions since these equilibration times or pulse series are long enough that were a reaction to occur, it will consume adsorbed gases to give multiple adsorption and situations where the amounts dosed are different from the amounts existent on the surface. E.g., in the prior art methods, a non-reactive room temperature must be used when gases like hydrogen or carbon monoxide are used with metal surfaces. This constraint of the measurement conditions to non-reactive conditions has the defect that the measured quantity does not have a direct relationship with the (concentration of) active sites where reactants under reaction conditions react, thus, the measurement may be inaccurate.
In chemical kinetics, the adsorption rate is calculated by the chemical kinetics expression:rads=k′adsCA(C0−CA,ads),where rads is the adsorption rate, k′ads is the adsorption rate constant per active site, CA is the concentration (or pressure) of the adsorbing gas, C0 is the active site concentration, and CA,ads is the surface species concentration. The per site adsorption rate constant, k′ads, used in the equation above differs by a multiplication factor equal to the active site concentration from the adsorption rate constant per unit weight powder (or per unit volume reactor, depending on the units used for the active site concentration) that is more commonly used in chemical kinetics texts. It is another defect of prior art methods that the active site concentration in this expression is not directly measured. Instead, the prior art methods first measure an entire surface area independently of the chemical kinetics expression, and then assume that this measured quantity is the same as the active site concentration in the kinetic expression when it is used in calculating the adsorption rate. Since it need not be the case that this entire surface area is the same as the active site concentration, C0, in the kinetics expression, the result may be inaccurate.
U.S. Pat. No. 5,264,183 issued to Ebner and Gleaves and U.S. Pat. No. 5,376,335 issued to Gleaves described pulsed valve apparatuses that are used to study the elution sequence of the reaction intermediates in a catalyzed reaction using the response curves of product gases. These apparatuses, reportedly used for the analysis of the sequence in which the reaction steps occur, have the defect that they cannot measure the active site concentration of the catalysts by the principle developed in this invention because they have no provisions for individual gas pulses to make detectable changes to the surface concentration. Contrariwise, the objects of the '183 and '335 patents require conditions where the amount of active sites of the catalyst powder sample is far more than the molecules in a gas pulse used, which makes it not possible to obtain the active site concentration of the catalysts because each gas pulse can then only make a negligible change to the surface concentration on the catalyst. Furthermore, the use of a pulsed gas technique requires the gas to be delivered rapidly, but the apparatuses described in the '183 and '335 patents include a mixing chamber or narrow channels or “zero-volume manifold” between the valve and reactor, which leads to undesirable pulse broadening.
A recent variant of the pulse or flow chemisorption method that used the apparatus described in the '335 patent used a series of similarly sized gas pulses that were injected until a change in the transient response (response curve) of a gas (either that injected or a product gas) showed that a change in the state of a catalyst had been effected by the many pulses. The amount of gas in this long series of same size gas pulses, and the modeling of the change in the shape or moments of the response curve caused by these, with the procedure experimentally controlled so that it needed very many pulses to cause a change in the response curve, was said useable to determine the amount of active sites. This method also has the defect that it uses the repeated pulsing of many intermittent pulses, which provides for adsorption on the entire surface or multiple adsorption on the same site.
In this invention, the term “gas diffusivity” refers to the effective diffusivity that is used when a gas-solid two phase system is viewed as a pseudo-homogeneous one phase medium. This is generally expressed as the diffusivity times porosity divided by tortuosity. “Intraparticle diffusivity” refers to the gas diffusivity inside a porous solid. The speed with which a gas can reach and exit the inside of a porous solid is an important kinetic parameter in many uses of porous powders. The intraparticle diffusivity is the measure of this property. Readily available techniques for measuring the intraparticle diffusivity in porous materials include the methods of permeability, Wilke-Kallenbach, time lag, sorption rate measurement, efficiency factor, frequency response, chromatographic methods, pulsed-field gradient NMR, and quasielastic neutron scattering, which are described in standard texts on diffusion in porous solids. However, none of these techniques have received widespread use because they either need large crystalline samples or expensive instruments or require very extensive measurements or are not precise. There is a need for an apparatus and a method that is simple, convenient to use, and precise enough, which is provided by this invention.
Previously, a pulsed valve apparatus of the type described in the '183 patent had been used for measuring the configuration diffusivity of strongly interacting gases in a zeolitic microporous powder packed bed by measuring the transient response (response curve) that elutes from the packed bed and using reactor simulation and regression to estimate the diffusivity. However, this was designed for use with the type of microporous powder and gas in which the gas interacts with the walls of the pores inside the powder. Generally, this interaction is one that causes the gas to stick onto the wall and because of this sticking interaction, it can only move slowly inside the pores or along the wall while stuck to the wall. This interaction has the effect that the elution of the gas from the packed bed is quite slow. Then, to avoid the appearance of response curves that are too much broadened, packed beds that are very short, generally less than 5 mm, are used. This has the disadvantage that with powders and gases where the gas does not interact with the walls except for collisions, in which there is no pulse broadening due to an interaction between the gas and wall, there is only limited precision in the measurement of the intraparticle diffusivity. There is a need for an apparatus and a method that can give improved precision, which is provided by this invention.
The measurement technique used in this invention is part of the art generally referred to as “relaxation kinetics”. Relaxation kinetics is implemented by a device that makes a sharp change, e.g. as a pulse or step injection, in the concentration of a molecular species delivered to a sample. Phenomena amendable to study can be limited by degradation of the sharpness of the pulse or step edge delivered to the sample because the information on these phenomena are extracted from the further change to the pulse or step edge shape, but if the “sharp” pulse or step edge is not sharp but occurs over a time extent longer than the characteristic time of the phenomenon, the further change due to the phenomenon will be small and cannot be reliably determined from the experimental data.
Prior art devices to transmit gas inputs from orifice openings to a sample under vacuum have the defect that they include at least a narrow channel and may further include a mixing chamber or some other protrusions between the point of injection and the sample. These include the devices described in the above '183 and '335 patents. When the gas delivery is performed under very low pressure conditions, these devices are flawed because at low pressures, broadening of a pulse or step edge is determined by the frequency of gas collisions with the channel wall or any added surfaces. Thus, the use of a narrow channel or mixing chamber or added solid surface is a fundamental defect because these increase the frequency of gas-solid collisions and cause pulse or edge broadening. This invention is also directed towards a device to deliver gas inputs from orifice openings to a sample under vacuum that has an increased rate of transmission and thus less degradation of the sharpness with which attached injection devices can make such sharp changes in concentration.
This invention is also an improvement over prior devices in simplifying the mathematical modeling for the quantitative extraction of information by parameter fitting. The kinetics parameters in the kinetics expressions can be extracted when a complete mathematical description of the flow of the molecular species or gases can be made. Here, the space where the flow of the gas occurs, which includes the space between a mechanical device that makes a concentration change and a device that removes the gas, and includes the catalyst powder and a detector means, is called the flow path. A complete mathematical description can be made when the geometry of the flow path is well defined and maintained under Knudsen flow conditions. This invention is further directed towards increasing the accuracy and tractability of a mathematical representation of the flow path by the use of a cylindrical geometry and means for a rapid expansion of gas input to a low pressure and Knudsen flow conditions.